1. Field of the Invention
This invention relates to a heat-sensitive material useful as a NTC (Negative Temperature Coefficient) material, which exhibits a decrease in electrical resistance with increasing temperature.
2. Description of the Related Art
Heat-sensitive elements that can be used as an over-charge (over voltage) protective element for a lithium ion secondary battery include a PTC (positive temperature coefficient) element, a bimetal element, and a thermal fuse.
These over-charge protective elements have the following disadvantages. Since the over-charge protective element is assembled in series to a charging circuit, it consumes power to decrease the capacity of a secondary battery. When an over-charge occurs actually, it takes time for both a lithium ion secondary battery and the over-charge protective element to show a change in appearance.
It is hence conceivable that an NTC element and a resistor R (e.g., a miniature bulb) are connected in parallel to a lithium ion secondary battery as shown in FIG. 1. If an over-charge takes place, the lithium ion secondary battery generates heat, and the NTC element trips and reduces its resistance due to the heat. It follows that a current flows through the NTC element and the resistor R thereby to shut down the charging current to the secondary battery.
The lowest resistivity reached by conventional NTC ceramics (e.g., about 10 Ω) is insufficient to shut down an overcharging current. Further, since the NTC characteristics of NTC ceramics are reversible, the protective element, after once having reduced its resistance, increases its resistance as it cools. It follows that the charging current is again allowed to flow through the lithium ion secondary battery. This means that a battery that should not be used may be used again, which is problematic for safety. For this reason, the conventional NTC ceramics are not fit for use as an over-charge protective element.